Electron multiplier



g- 1941- F. J. ca. VAN DEN BOSCH 2,254,128

ELECTRON MULTIPLIER Filed June 1, 1939 J. G. \/A

H C S O R O T N E M w w w all .I l m j w W M'III H %flfi ln fl l||l|||l WW i. i fi Y 1 j w m fl F m 5 HA s Y T T A Patented Aug. 26, 1941 marlin snares enrsnr owner 1 v V l 2,254,128 I ELECTRON Il/IULTIPLIER Francois Joseph Gerard Van den Bosch, London, England, assignor to Vacuum-Science Products Limited, London, England, a British company Application June 1, 1939, Serial No. 276,883 In Great'Britain June 2, 1938 9 Claims.

This invention relates to electron-multipliers comprising an electron-emitting cathode, one or more secondary electron-emitting electrodes arranged in an envelope in such manner that electrons emitted by the cathode are caused to impinge on a secondary electron-emitting electrode and liberate other electrons which impinge upon the next secondary electron-emitting electrode, when more than one, and so on, to a final collector.

According to a feature of this invention there is provided an electron-multiplier having one or more secondary electron-emitting electrodes each of which is formed of a perforated metal plate.

According to another feature of the invention there is provided an electron-multiplier comprising one or more secondary electron-emitting electrodes each of which has holes for the passage of electrons, which holes are bounded by. a wall, of which at least a part on which approaching electrons impinge is inclined to the path of these approaching electrons. Preferably, holes are of diminishing cross-section in the direction of travel of the electrons.

Another feature of the invention consists in the provision of a plurality of secondary electron-emitting electrodes arranged one behind the other in spaced relationship and preferably the holes are staggered in successive secondary electron-emitting electrodes.

One specific embodiment of electron-multiplier according tothe invention is shown by way of example in the accompanying drawing, in which:

Figure 1 is a diagrammatic elevation of the multiplie- Figure 2 is a section on line 22 of Figure 1;

Figure 3 is a front viewof a secondary electrcn emitting electrode on an enlarged scale;

' Figure 4 is a section on line 4-4 of Figure 3;

Figure 5 is a front View of a modified form of secondary electron-emitting electrode;

Figure 6 is a section on line 66 of Figure 5, and,

Figure '7 is a detail view showing the provision of a heater for the secondary electron emitting electrode.

Referring to the drawing, there is showna source of primary electrons consisting of a cathode H3. For the control of the primary electrons there is provided a cylindrical electrode ll around the cathode and a control grid I 2 extending across but spaced from an open end of the cylindrical electrode I I. An accelerator gridouter layer coated with oxide of cesium or other alkali metal. These electrodes are provided with a plurality of holes Ill. The holes are conveniently formed by a punching operation such that v the metal bounding each hole is deformed to form a conical wall 20 for the hole. The construction is such that thearea of the hole at the front of the electrode is greater than the area of the hole at the rear of the electrode, for example, in the proportion of approximately 5:3. The holes, considered according to their largest dimension, may occupy about 15 percent of the area of the electrode. Y

The secondary electron-emitting electrodes are mounted in parallel relationship one behind another so that the holes in them are of decreasing area in the direction of travel of electrons. The electrodes are also arranged with the holes in them staggered with respect to those of an adjacent electrode. Each 'of the secondary electron-emitting electrodes has a peripheral flange 2 i extending rearwardly of the electrode and the electrodes I5, 16, I1 and i8 are progressively smaller in diameter so that they nest one'within the other with the flange of one electrode surrounding the space between that electrode and the next.

Close to the secondary electron-emitting electrode i8, for example, about /2 mm. from it, there is a collecting electrode 23 constructed as a Wire grid or mesh and at the remote side of this collector electrode there is a final secondary electron-emitting electrode 24. The cathode Ii), associated electrodes H, I 2 and l3and reflector Mare carried in a pinch 25 mounted in one end of an envelope 25 and the collector electrode 23 and secondary electron-emitting electrodes'are carried in another pinch 21 mounted in the 0pposite'end of the envelope 26. The'connections 28 to the electrodes 15, 16,, I1 and I8 indicated in Figure 2 are distributed around the periphery of these electrodes and are spaced apart in the pinch 27 in order to minimise the inter-electrode capacity. I

In use of the electronmultiplier,

progressively increasing positive potentials with respect to the cathode ID are applied to the secondary electron-emitting electrodes l5, l6, ll, l8 and 24 and the collector 23. Primary electrons from the cathode Iii are directed onto the first secondary electron-emitting electrode I under the control of the electrodes ll, l2 and I3 and liberate secondary electrons which pass through the holes in this electrode to the next secondary electron-emitting electrode l6, and so on, to the final secondary electron-emitting electrode 24. The secondary emission from this electrode 24 is received on the collector 23, from which the output is taken. The flanges 2|, because they have the same operating potential as the electrodes of which they form part, and extend rearwardly of these electrodes towards the next electrode of higher potential, constitute electrostatic focussing means serving to concentrate the electrons in a beam through the secondary electron-emitting electrodes towards the collector 23.

Electrons travelling towards a secondary electron-emitting electrode impinge upon the inclined or conical surfaces 20 constituted by the walls forming the holes of the secondary electron-emitting electrodes and thus strike the electrode at an angle well suited for the production of secondary electrons and for the liberation of .these electrons in a direction through the holes towards the next electrode. Instead of employing as the secondary electron-emitting electrodes, discs formed with a plurality of holes as described with reference to Figures 3 and 4, they may be constituted by discs of the form shown in Figures 5 and 6. In this alternative construction the electrode consists of a metal disc formed with a plurality of narrow slots 30. These slots are conveniently produced by a punching operation, displacing the metal to provide the slots in such a manner as to produce inclined walls 3! for the same purpose as the conical surfaces 20 of the construction described with reference to Figures 2 and 3.

In order to assist the liberation of secondary electrons from the electrodes IE to l8 indirect heating means may be provided for these electrodes to raise them in temperature but not sufficiently high to produce appreciable primary emission. For this purpose the electrodes may be indirectly heated by means of a heater coil 32 as shown in Figure 7 carried on the flange 2|.

It will be understood that the invention is not restricted to the specific embodiments hereinbefore described and in particular, a transparent or apaque photo-electric cathode may be used instead of a thermionic cathode. Moreover, electrodes for performing other electronic functions may be incorporated in the envelope in addition to the electrodes for obtaining electron multiplication.

I claim: 7

1. An electron multiplier comprising a primary cathode, a secondary electron-emitting electrode for emitting secondary electrons under bombardment by other electrons and having holes for the passage of electrons, and a collector electrode for the secondary electrons, which holes are each bounded by a wall of which a part on which approaching electrons impinge is inclined to the path of the approaching electrons.

2. An electron multiplier comprising a primary cathode, a secondary electron-emitting electrode for emitting secondary electrons under bombardment by other electrons, which secondary electron-emitting electrode is formed with holes for the passage of electrons and. diminishing in crosssection in the direction of travel of the secondary electrons and a collector electrode for the secondary electrons.

3; An electron multiplier comprising a primary electron-emitting electrode, a plurality of secondary electron-emitting electrodes mounted one behind another and spaced progressively from said primary electron-emitting electrode, which secondary electron-emitting electrodes are formed with holes for the passage of electrons, a final collector electrode for the secondary electrons, which secondary electron-emitting electrodes are progressively smaller in size in the direction of travel of electrons, and are each provided with a peripheral flange extending therefrom to surround the space between the electrode and the next following smaller electrode.

4. An electron multiplier comprising a primary electron-emitting electrode, a plurality of secondary electron-emitting electrodes mounted one behind another and spaced progressively from said primary electron-emitting electrode, which secondary electron-emitting electrodes are formed with holes for the passage of electrons, a final collector electrode for the secondary electrons, which secondary electron-emitting electrodes are each provided with a peripheral flange extending therefrom to surround the space between the electrode and the next following electrode, and means provided on the flange of each secondary electron-emitting electrode to indirectly heat the electrode to assist the secondary emission.

5. An electron multiplier comprising a primary electron-emitting electrode, a plurality of secondary electron-emitting electrodes mounted one behind another and spaced progressively from said primary electron-emitting electrode, which secondary electron-emitting electrodes are formed with holes for the passage of electrons, a final collector electrode for the secondary electrons, which secondary electron-emitting electrodes are progressively smaller in size in the direction of travel of electrons and are each provided with a peripheral flange extending therefrom to surround the space between the electrode and the next following smaller electrode, and means carried on the flange of each secondary electron-emitting electrode for indirectly heating the electrode to assist the secondary emission.

6. An electron multiplier comprising a primary cathode, a secondary electron-emitting electrode for emitting electrons under bombardment by other electrons, which secondary electron-emitting electrode is formed with holes for the passage of electrons and diminishing in cross-section in the direction of travel of the secondary electrons, which holes according to their largest dimension occupy approximately 15 per cent of the area of the electrode in which they are situated, and a collector electrode for the secondary electrons.

7. An electron multiplier comprising a primary cathode, a secondary electron-emitting electrode for emitting electrons under bombardment by other electrons, which secondary electron-emitting electrode is formed with holes for the passage of electrons and diminishing in cross-section in the direction of travel of the secondary electrons, which holes are approximately 1 mm. deep and the proportion of the area of the top of the hole to the area at the bottom of the hole is approximately 5:3, and a collector electrode for the secondary electrons.

8. An electron multiplier comprising a primary cathode, a secondary electron-emitting electrode for emitting electrons under bombardment by other electrons, which secondary electronemitting electrode is formed with holes for the passage of electrons, which holes are in the form of narrow slots extending across the electrode and diminishing in cross-section in the direction of travel of the secondary electrons and a collector for the secondary electrons.

9. An electron multiplier comprising a primary electron-emitting electrode, a plurality of second- 

